I have read a couple of posts here about dealing with "Humidity". It is important to understand what "Relative Humidity" (RH) is and have an idea how it is measured. There seems to be some confusion about RH as the temperature changes. It's true that the air can hold more moisture when the temperature increases, but most us are measuring RH and the readings are correct for the given temperatures. 50% is 50% is 50% at any given temperature.
Relative humidity is the ratio of the current absolute humidity to the highest possible absolute humidity (which depends on the current air temperature). A reading of 100 percent relative humidity means that the air is totally saturated with water vapor and cannot hold any more, creating the possibility of rain.
Absolute humidity is the mass of water vapor divided by the mass of dry air in a volume of air at a given temperature. The hotter the air is, the more water it can contain. Absolute humidity is expressed as grams of moisture per cubic meter of air (g/m3).We need to make sure we are monitoring the Relative Humidity for our guitars, whether that is in their cases (like I have to do) or in the room where they are left open for ease of use. I live in SE Asia and the relative humidity always seems to hover around 80% for rooms that are not air-conditioned even when the weather report says 60%. I believe that is a result of the house and furnishings absorbing moisture and releasing it. So, I have a very hot and humid environment to deal with. There are many of you that have the opposite problem and have to hydrate your guitars so they don't dry out.
I use a digital hygrometer in my cases that can be calibrated and measures the Relative Humidity. They are very accurate and can easily be calibrated and adjusted.
I am able to keep my guitars at around 53% +/- 2% at 82 degrees fahrenheit and at 75 degrees in air-conditioned rooms, where I practice and play, the relative humidity will drop to around 48%. When the guitar is placed back into the case and stored in a room that isn't air-conditioned the RH is staying at the 53%.
I tried silica gels in 10 gram size and also would take four or five of the 5 gram size and attach five each to two thin strips of gaffers tape and place in the sound hole along with the 10 gram at the body and headstock. They just were not getting the job done for me with the RH hovering around 60-65%. I purchased Boveda 49% High Absorption packs along with the cloth holders and they are working great. I think Boveda manufactures the ones sold by Taylor and D'Addario. I have no experience with the 49% Boveda packs that are supposed to hydrate the guitar case.
There are basically three types of hygrometers:
- Dry / Wet Bulb
- Mechanical
- Digital
Hygrometers should be calibrated regularly, but definitely annually. You can do this by placing the hygrometer into a large zip lock with a cup of salt water and leaving it at room temperature for at least ten hours. It should read 75%. I had one hygrometer that was sold for guitar cases that wasn't adjustable and it ended up not being very accurate. If you are going to purchase a new one I would make sure that the hygrometer can be adjusted whether a mechanical one or digital. Save yourself the headache of having to attach a note showing it reads xx% low or high.
Check out the following link if you want more info.
Worboys, Jenni. "What Is a Hygrometer and How Does It Work?" ThoughtCo, Jun. 22, 2018, thoughtco.com/what-is-a-hygrometer-3444417.
